I would think you would need to follow the registration laws of the Country that issued your passport. It's makes coming home easier. Customs does check registrations.

bubb2, thanks, the problem with your advice is that as I said before, Canada (my passport issuer) does not require licensing for a sailboat without an engine and the registration is optional for any pleasure craft. This means that according to the country law I do not need to do either.

Registration (Documentation in US) is a title but would other proof of ownership such as USCG bill of sale or notarized (even legalized) bill of sale be accepted as a proof of ownership in lieu of registration?

Does a pleasure craft have to fly a flag? Flying a Canadian flag does require registration. Which international law states that?